1. Field of the Invention
This invention relates to a mechanical compensation type zoom lens system capable of improving performances of large diameter lenses, wide angle lenses and highly variable magnification zoom lenses, and more particularly to a zoom lens system capable of effectively correcting distortional aberration greatly varying from minus to plus on the wide angle side.
2. Description of the Prior Art
As has been well known, a magnitude of a value of distortional aberration is determined mostly by an angle of incidence of light .omega., focal lengths of groups of lenses apart from an aperture and shapes of the lenses.
In the case of the zoom lens system, as the zooming is effected from the wide angle side to the telephoto side, the angle of incidence of light .omega. is decreased, whereby the value of distortional aberration should necessarily be decreased as well.
However, in reality, the distortional aberration is greatly varied from minus to plus in the vicinity of the wide angle (Refer to FIG. 3). This positive distortional aberration is a so-called spool shaped and highly notable.
The inventor of the present invention proposes a zooming lens system according to the present invention on the basis of the recognition that the variation of distortional aberration in the vicinity of the wide angle is principally caused by a change of entrance pupil position due to zooming.
More specifically, there has heretofore been known that, in the case where the zooming is effected by two groups of lenses, the zooming contemplates to vary the magnification while the conjugate length of the two groups of lenses is held constant, so that relative positions (movement loci) of the two groups of lenses through the zooming may be given by a solution of a quadratic equation. Since the change of entrance pupil position is also determined by this solution, characteristic fluctuations of the distortional aberration cannot be corrected as it is.
There has been tried to correct this distortional aberration by shapes of lenses, which, however, necessitates other aberrations including spherical aberration, curvature of field and the like to be sacrificed to a considerable extent.
In general, during the zooming from the wide angle side W to the telephoto side T as shown in FIG. 1, a zoom lens system consisting of two movable lens groups including a first movable lens group of divergence and a second movable lens group of convergence draws loci indicated by solid lines in FIG. 1, with the first movable lens group l.sub.1 moving rearwardly and the second movable lens group l.sub.2 moving forwardly. In this case, the entrance pupil position P moves rearwardly, drawing a locus indicated by two-dot chain lines in FIG. 1.
Because of this, although an incident angle of view .omega.' is small in the vicinity of the wide angle (1.5.times..about.2.0.times. of zoom ratio) as shown in W.sub.1 of FIG. 1, h' is large, thus largely contributing to the generation of positive distortional aberration.
To resolve this problem, it is necessary to find a zooming method in which the fluctuation value of entrance pupil position P on the wide angle side due to zooming is small.